The present invention relates to a down-the-hole hammer drill having a reverse circulation system and to a drill bit for such a drill.
A down-the-hole hammer drill employs a drill pipe to which a drill bit is mounted, and wherein a reciprocating piston is situated in a drill casing immediately above the bit for imparting repeated blows to the bit.
A reverse circulation system involves the downward circulation of a flushing medium, such as air, to a front cutting face of the bit, whereupon the air flows laterally across the putting face and eventually up through a center passage in the bit and hammer to the ground surface. Cuttings become entrained within the air as the air passes across the cutting face of the bit and are conducted to the surface along with the air and are analyzed for mineral content. Also, the air serves to cool and flush cutter elements mounted in the cutting face.
Prior art drills of that type are disclosed, for example, in U.S. Pat. No. 4,321,974 issued Mar. 30, 1982 and U.S. Pat. No. 4,819,746 issued Apr. 11, 1989. That drill includes a drill bit mounted within a driver sub, the latter serving to transmit rotation from a drill string to the drill bit. A reciprocating piston is situated immediately above the bit and is reciprocated by an air flow introduced through the drill string. Air exhausted from the piston is conducted downwardly through a space formed between the drill bit and a driver sub. The driver sub is enlarged conically at its lower end to encircle a rear shoulder of the drill bit. Air is able to leak out between that shoulder and the enlarged conical end. In an attempt to resist the escape of air upwardly through an annular gap formed between the driver sub and a wall of the hole being drilled, the bit is provided with longitudinal bores and transverse bores. The longitudinal bores include air inlets located at a rear shoulder of the bit, and air outlets opening into the cutting face. The lateral bores include air inlets opening at the rear shoulder, and air outlets communicating with the center passage at a location above the cutting face. The air exiting the longitudinal and lateral bores is intended to create a suction for resisting the escape of air upwardly through the annular gap.
Notwithstanding the presence of the longitudinal and lateral bores, some air may still escape upwardly through the annular gap. Moreover, there exists the possibility of the inlets of the bores becoming clogged with dirt if the bores are not effectively isolated from the surrounding environment.
Another shortcoming associated with such drills involves difficulties in forming the fluid passages in the drill bit which can lead to undesirably high manufacturing costs.
It would be desirable to provide a hammer drill which alleviates the above-described problems and yet which can be economically manufactured.